diff mbox series

[2/5] nptl: Add rseq registration

Message ID 9c58724d604e160ebda5f667331fa41416c0d12b.1638798186.git.fweimer@redhat.com
State Superseded
Delegated to: Szabolcs Nagy
Headers show
Series Extensible rseq support for glibc | expand

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent

Commit Message

Florian Weimer Dec. 6, 2021, 1:46 p.m. UTC
The rseq area is placed directly into struct pthread.  rseq
registration failure is not treated as an error, so it is possible
that threads run with inconsistent registration status.

<sys/rseq.h> is not yet installed as a public header.

Co-Authored-By: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 csu/libc-tls.c                              |   6 +
 nptl/descr.h                                |   4 +
 nptl/pthread_create.c                       |  13 +
 sysdeps/nptl/dl-tls_init_tp.c               |   8 +-
 sysdeps/unix/sysv/linux/Makefile            |   9 +-
 sysdeps/unix/sysv/linux/aarch64/bits/rseq.h |  43 ++++
 sysdeps/unix/sysv/linux/arm/bits/rseq.h     |  83 +++++++
 sysdeps/unix/sysv/linux/bits/rseq.h         |  29 +++
 sysdeps/unix/sysv/linux/mips/bits/rseq.h    |  62 +++++
 sysdeps/unix/sysv/linux/powerpc/bits/rseq.h |  37 +++
 sysdeps/unix/sysv/linux/rseq-internal.h     |  45 ++++
 sysdeps/unix/sysv/linux/s390/bits/rseq.h    |  37 +++
 sysdeps/unix/sysv/linux/sys/rseq.h          | 174 +++++++++++++
 sysdeps/unix/sysv/linux/tst-rseq-nptl.c     | 260 ++++++++++++++++++++
 sysdeps/unix/sysv/linux/tst-rseq.c          |  64 +++++
 sysdeps/unix/sysv/linux/tst-rseq.h          |  57 +++++
 sysdeps/unix/sysv/linux/x86/bits/rseq.h     |  30 +++
 17 files changed, 958 insertions(+), 3 deletions(-)
 create mode 100644 sysdeps/unix/sysv/linux/aarch64/bits/rseq.h
 create mode 100644 sysdeps/unix/sysv/linux/arm/bits/rseq.h
 create mode 100644 sysdeps/unix/sysv/linux/bits/rseq.h
 create mode 100644 sysdeps/unix/sysv/linux/mips/bits/rseq.h
 create mode 100644 sysdeps/unix/sysv/linux/powerpc/bits/rseq.h
 create mode 100644 sysdeps/unix/sysv/linux/rseq-internal.h
 create mode 100644 sysdeps/unix/sysv/linux/s390/bits/rseq.h
 create mode 100644 sysdeps/unix/sysv/linux/sys/rseq.h
 create mode 100644 sysdeps/unix/sysv/linux/tst-rseq-nptl.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-rseq.c
 create mode 100644 sysdeps/unix/sysv/linux/tst-rseq.h
 create mode 100644 sysdeps/unix/sysv/linux/x86/bits/rseq.h

Comments

Mathieu Desnoyers Dec. 6, 2021, 4:53 p.m. UTC | #1
----- On Dec 6, 2021, at 8:46 AM, Florian Weimer fweimer@redhat.com wrote:

> The rseq area is placed directly into struct pthread.  rseq
> registration failure is not treated as an error, so it is possible
> that threads run with inconsistent registration status.
> 
> <sys/rseq.h> is not yet installed as a public header.
> 
> Co-Authored-By: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> ---

[...]

> --- a/csu/libc-tls.c
> +++ b/csu/libc-tls.c
> @@ -112,6 +112,12 @@ __libc_setup_tls (void)
>   size_t tcb_offset;
>   const ElfW(Phdr) *phdr;
> 
> +  /* libc.so with rseq has TLS with 32-byte alignment.  Static dlopen
> +     requires at least 32-byte alignment as well, otherwise loading
> +     libc.so will always fail.  */
> +  if (max_align < 32)
> +    max_align = 32;

Now that the rseq area is move into struct pthread, do we still require a
32-byte alignment on the TLS ?

[...]
> --- a/nptl/pthread_create.c
> +++ b/nptl/pthread_create.c

[...]
> 
> +     rseq TLS is still registered at this point.  Rely on implicit
> +     unregistration performed by the kernel on thread teardown.  This is not a
> +     problem because the rseq TLS lives on the stack, and the stack outlives

I think we need to update this comment, because it does not make sense anymore to
discuss the placement of the rseq TLS now that it moves to struct pthread.

Is struct pthread placed on the thread stack ?

> +     the thread.  If TCB allocation is ever changed, additional steps may be
> +     required, such as performing explicit rseq unregistration before
> +     reclaiming the rseq TLS area memory.  It is NOT sufficient to block
> +     signals because the kernel may write to the rseq area even without
> +     signals.
> +

Thanks,

Mathieu
Mathieu Desnoyers Dec. 6, 2021, 4:59 p.m. UTC | #2
----- On Dec 6, 2021, at 8:46 AM, Florian Weimer fweimer@redhat.com wrote:
[...]
> @@ -406,6 +407,9 @@ struct pthread
>   /* Used on strsignal.  */
>   struct tls_internal_t tls_state;
> 
> +  /* rseq area registered with the kernel.  */
> +  struct rseq rseq_area;

The rseq UAPI requires that the fields within the rseq_area
are read-written with single-copy atomicity semantics.

So either we define a "volatile struct rseq" here, or we'll need
to wrap all accesses with the proper volatile casts, or use the
relaxed_mo atomic accesses.

Thanks,

Mathieu


> +
>   /* This member must be last.  */
>   char end_padding[];
>
Florian Weimer Dec. 6, 2021, 5:10 p.m. UTC | #3
* Mathieu Desnoyers:

>> --- a/csu/libc-tls.c
>> +++ b/csu/libc-tls.c
>> @@ -112,6 +112,12 @@ __libc_setup_tls (void)
>>   size_t tcb_offset;
>>   const ElfW(Phdr) *phdr;
>> 
>> +  /* libc.so with rseq has TLS with 32-byte alignment.  Static dlopen
>> +     requires at least 32-byte alignment as well, otherwise loading
>> +     libc.so will always fail.  */
>> +  if (max_align < 32)
>> +    max_align = 32;
>
> Now that the rseq area is move into struct pthread, do we still require a
> 32-byte alignment on the TLS ?

Yes, we can remove that.  In fact, I increased TCB_ALIGNMENT
specifically for that.

>> --- a/nptl/pthread_create.c
>> +++ b/nptl/pthread_create.c
>
> [...]
>> 
>> +     rseq TLS is still registered at this point.  Rely on implicit
>> +     unregistration performed by the kernel on thread teardown.  This is not a
>> +     problem because the rseq TLS lives on the stack, and the stack outlives
>
> I think we need to update this comment, because it does not make sense
> anymore to discuss the placement of the rseq TLS now that it moves to
> struct pthread.
>
> Is struct pthread placed on the thread stack ?

Yes, it is, along with the TLS data, which I thought the current wording
was still fine.

Thanks,
Florian
Florian Weimer Dec. 6, 2021, 5:14 p.m. UTC | #4
* Mathieu Desnoyers:

> ----- On Dec 6, 2021, at 8:46 AM, Florian Weimer fweimer@redhat.com wrote:
> [...]
>> @@ -406,6 +407,9 @@ struct pthread
>>   /* Used on strsignal.  */
>>   struct tls_internal_t tls_state;
>> 
>> +  /* rseq area registered with the kernel.  */
>> +  struct rseq rseq_area;
>
> The rseq UAPI requires that the fields within the rseq_area
> are read-written with single-copy atomicity semantics.
>
> So either we define a "volatile struct rseq" here, or we'll need
> to wrap all accesses with the proper volatile casts, or use the
> relaxed_mo atomic accesses.

Under the C memory model, neither volatile nor relaxed MO result in
single-copy atomicity semantics.  So I'm not sure what to make of this.
Surely switching to inline assembly on all targets is over the top.

I think we can rely on a plain read doing the right thing for us.

Thanks,
Florian
Mathieu Desnoyers Dec. 6, 2021, 6:52 p.m. UTC | #5
[ Adding other kernel rseq maintainers in CC. ]

----- On Dec 6, 2021, at 12:14 PM, Florian Weimer fweimer@redhat.com wrote:

> * Mathieu Desnoyers:
> 
>> ----- On Dec 6, 2021, at 8:46 AM, Florian Weimer fweimer@redhat.com wrote:
>> [...]
>>> @@ -406,6 +407,9 @@ struct pthread
>>>   /* Used on strsignal.  */
>>>   struct tls_internal_t tls_state;
>>> 
>>> +  /* rseq area registered with the kernel.  */
>>> +  struct rseq rseq_area;
>>
>> The rseq UAPI requires that the fields within the rseq_area
>> are read-written with single-copy atomicity semantics.
>>
>> So either we define a "volatile struct rseq" here, or we'll need
>> to wrap all accesses with the proper volatile casts, or use the
>> relaxed_mo atomic accesses.
> 
> Under the C memory model, neither volatile nor relaxed MO result in
> single-copy atomicity semantics.  So I'm not sure what to make of this.
> Surely switching to inline assembly on all targets is over the top.
> 
> I think we can rely on a plain read doing the right thing for us.

AFAIU, the plain read does not prevent the compiler from re-loading the
value in case of high register pressure.

Accesses to rseq fields such as cpu_id need to be done as if those were
concurrently modified by a signal handler nesting on top of the user-space
code, with the particular twist that blocking signals has no effect on
concurrent updates.

I do not think we need to do the load in assembly. I was under the impression
that both volatile load and relaxed MO result in single-copy atomicity
semantics for an aligned pointer. Perhaps Paul, Peter, Boqun have something
to add here ?

Thanks,

Mathieu
Florian Weimer Dec. 6, 2021, 7:03 p.m. UTC | #6
* Mathieu Desnoyers:

> [ Adding other kernel rseq maintainers in CC. ]
>
> ----- On Dec 6, 2021, at 12:14 PM, Florian Weimer fweimer@redhat.com wrote:
>
>> * Mathieu Desnoyers:
>> 
>>> ----- On Dec 6, 2021, at 8:46 AM, Florian Weimer fweimer@redhat.com wrote:
>>> [...]
>>>> @@ -406,6 +407,9 @@ struct pthread
>>>>   /* Used on strsignal.  */
>>>>   struct tls_internal_t tls_state;
>>>> 
>>>> +  /* rseq area registered with the kernel.  */
>>>> +  struct rseq rseq_area;
>>>
>>> The rseq UAPI requires that the fields within the rseq_area
>>> are read-written with single-copy atomicity semantics.
>>>
>>> So either we define a "volatile struct rseq" here, or we'll need
>>> to wrap all accesses with the proper volatile casts, or use the
>>> relaxed_mo atomic accesses.
>> 
>> Under the C memory model, neither volatile nor relaxed MO result in
>> single-copy atomicity semantics.  So I'm not sure what to make of this.
>> Surely switching to inline assembly on all targets is over the top.
>> 
>> I think we can rely on a plain read doing the right thing for us.
>
> AFAIU, the plain read does not prevent the compiler from re-loading the
> value in case of high register pressure.
>
> Accesses to rseq fields such as cpu_id need to be done as if those were
> concurrently modified by a signal handler nesting on top of the user-space
> code, with the particular twist that blocking signals has no effect on
> concurrent updates.
>
> I do not think we need to do the load in assembly. I was under the impression
> that both volatile load and relaxed MO result in single-copy atomicity
> semantics for an aligned pointer. Perhaps Paul, Peter, Boqun have something
> to add here ?

The C memory model is broken and does not prevent out-of-thin-air
values.  As far as I know, this breaks single-copy atomicity.  In
practice, compilers will not exercise the latitude offered by the memory
model.  volatile does not ensure absence of data races.

Using atomics or volatile would require us to materialize the thread
pointer, given the current internal interfaces we have, and I don't want
to do this because this is supposed to be performance-critical code.
The compiler barrier inherent to the function call will have to be
enough.  I can add a comment to this effect:

  /* This load has single-copy atomicity semantics (as required for
     rseq) because the function call implies a compiler barrier.  */

Thanks,
Florian
Cornea, Marius via Libc-alpha Dec. 6, 2021, 8:11 p.m. UTC | #7
On Mon, Dec 06, 2021 at 08:03:26PM +0100, Florian Weimer wrote:
> * Mathieu Desnoyers:
> 
> > [ Adding other kernel rseq maintainers in CC. ]
> >
> > ----- On Dec 6, 2021, at 12:14 PM, Florian Weimer fweimer@redhat.com wrote:
> >
> >> * Mathieu Desnoyers:
> >> 
> >>> ----- On Dec 6, 2021, at 8:46 AM, Florian Weimer fweimer@redhat.com wrote:
> >>> [...]
> >>>> @@ -406,6 +407,9 @@ struct pthread
> >>>>   /* Used on strsignal.  */
> >>>>   struct tls_internal_t tls_state;
> >>>> 
> >>>> +  /* rseq area registered with the kernel.  */
> >>>> +  struct rseq rseq_area;
> >>>
> >>> The rseq UAPI requires that the fields within the rseq_area
> >>> are read-written with single-copy atomicity semantics.
> >>>
> >>> So either we define a "volatile struct rseq" here, or we'll need
> >>> to wrap all accesses with the proper volatile casts, or use the
> >>> relaxed_mo atomic accesses.
> >> 
> >> Under the C memory model, neither volatile nor relaxed MO result in
> >> single-copy atomicity semantics.  So I'm not sure what to make of this.
> >> Surely switching to inline assembly on all targets is over the top.
> >> 
> >> I think we can rely on a plain read doing the right thing for us.
> >
> > AFAIU, the plain read does not prevent the compiler from re-loading the
> > value in case of high register pressure.
> >
> > Accesses to rseq fields such as cpu_id need to be done as if those were
> > concurrently modified by a signal handler nesting on top of the user-space
> > code, with the particular twist that blocking signals has no effect on
> > concurrent updates.
> >
> > I do not think we need to do the load in assembly. I was under the impression
> > that both volatile load and relaxed MO result in single-copy atomicity
> > semantics for an aligned pointer. Perhaps Paul, Peter, Boqun have something
> > to add here ?
> 
> The C memory model is broken and does not prevent out-of-thin-air
> values.  As far as I know, this breaks single-copy atomicity.  In
> practice, compilers will not exercise the latitude offered by the memory
> model.  volatile does not ensure absence of data races.

Within the confines of the standard, agreed, use of the volatile keyword
does not explicitly prevent data races.

However, volatile accesses are (informally) defined to suffice for
device-driver memory accesses that communicate with devices, whether via
MMIO or DMA-style shared memory.  The device-driver firmware is often
written in C or C++.  So doesn't this informal device-driver guarantee
need to also do what is needed for userspace code that is communicating
with kernel code?  If not, why not?

> Using atomics or volatile would require us to materialize the thread
> pointer, given the current internal interfaces we have, and I don't want
> to do this because this is supposed to be performance-critical code.
> The compiler barrier inherent to the function call will have to be
> enough.  I can add a comment to this effect:
> 
>   /* This load has single-copy atomicity semantics (as required for
>      rseq) because the function call implies a compiler barrier.  */

Agreed on the need to be very careful to avoid degrading performance on
fast paths!

							Thanx, Paul
Florian Weimer Dec. 6, 2021, 8:26 p.m. UTC | #8
* Paul E. McKenney via Libc-alpha:

>> The C memory model is broken and does not prevent out-of-thin-air
>> values.  As far as I know, this breaks single-copy atomicity.  In
>> practice, compilers will not exercise the latitude offered by the memory
>> model.  volatile does not ensure absence of data races.
>
> Within the confines of the standard, agreed, use of the volatile keyword
> does not explicitly prevent data races.
>
> However, volatile accesses are (informally) defined to suffice for
> device-driver memory accesses that communicate with devices, whether via
> MMIO or DMA-style shared memory.  The device-driver firmware is often
> written in C or C++.  So doesn't this informal device-driver guarantee
> need to also do what is needed for userspace code that is communicating
> with kernel code?  If not, why not?

The informal guarantee is probably good enough here, too.  However, the
actual accesses are behind macros, and those macros use either
non-volatile plain reads or inline assembler (which use
single-instruction naturally aligned reads).

THanks,
Florian
Cornea, Marius via Libc-alpha Dec. 6, 2021, 9:08 p.m. UTC | #9
On Mon, Dec 06, 2021 at 09:26:51PM +0100, Florian Weimer wrote:
> * Paul E. McKenney via Libc-alpha:
> 
> >> The C memory model is broken and does not prevent out-of-thin-air
> >> values.  As far as I know, this breaks single-copy atomicity.  In
> >> practice, compilers will not exercise the latitude offered by the memory
> >> model.  volatile does not ensure absence of data races.
> >
> > Within the confines of the standard, agreed, use of the volatile keyword
> > does not explicitly prevent data races.
> >
> > However, volatile accesses are (informally) defined to suffice for
> > device-driver memory accesses that communicate with devices, whether via
> > MMIO or DMA-style shared memory.  The device-driver firmware is often
> > written in C or C++.  So doesn't this informal device-driver guarantee
> > need to also do what is needed for userspace code that is communicating
> > with kernel code?  If not, why not?
> 
> The informal guarantee is probably good enough here, too.  However, the
> actual accesses are behind macros, and those macros use either
> non-volatile plain reads or inline assembler (which use
> single-instruction naturally aligned reads).

Agreed, a non-volatile plain read is quite dangerous in this context.

							Thanx, Paul
diff mbox series

Patch

diff --git a/csu/libc-tls.c b/csu/libc-tls.c
index 5515204863..0d8146c0fa 100644
--- a/csu/libc-tls.c
+++ b/csu/libc-tls.c
@@ -112,6 +112,12 @@  __libc_setup_tls (void)
   size_t tcb_offset;
   const ElfW(Phdr) *phdr;
 
+  /* libc.so with rseq has TLS with 32-byte alignment.  Static dlopen
+     requires at least 32-byte alignment as well, otherwise loading
+     libc.so will always fail.  */
+  if (max_align < 32)
+    max_align = 32;
+
   struct link_map *main_map = GL(dl_ns)[LM_ID_BASE]._ns_loaded;
 
   __tls_pre_init_tp ();
diff --git a/nptl/descr.h b/nptl/descr.h
index af2a6ab87a..92db305913 100644
--- a/nptl/descr.h
+++ b/nptl/descr.h
@@ -34,6 +34,7 @@ 
 #include <bits/types/res_state.h>
 #include <kernel-features.h>
 #include <tls-internal-struct.h>
+#include <sys/rseq.h>
 
 #ifndef TCB_ALIGNMENT
 # define TCB_ALIGNMENT 32
@@ -406,6 +407,9 @@  struct pthread
   /* Used on strsignal.  */
   struct tls_internal_t tls_state;
 
+  /* rseq area registered with the kernel.  */
+  struct rseq rseq_area;
+
   /* This member must be last.  */
   char end_padding[];
 
diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index bad9eeb52f..ea0d79341e 100644
--- a/nptl/pthread_create.c
+++ b/nptl/pthread_create.c
@@ -32,6 +32,7 @@ 
 #include <default-sched.h>
 #include <futex-internal.h>
 #include <tls-setup.h>
+#include <rseq-internal.h>
 #include "libioP.h"
 #include <sys/single_threaded.h>
 #include <version.h>
@@ -366,6 +367,9 @@  start_thread (void *arg)
   /* Initialize pointers to locale data.  */
   __ctype_init ();
 
+  /* Register rseq TLS to the kernel.  */
+  rseq_register_current_thread (pd);
+
 #ifndef __ASSUME_SET_ROBUST_LIST
   if (__nptl_set_robust_list_avail)
 #endif
@@ -571,6 +575,15 @@  out:
      process is really dead since 'clone' got passed the CLONE_CHILD_CLEARTID
      flag.  The 'tid' field in the TCB will be set to zero.
 
+     rseq TLS is still registered at this point.  Rely on implicit
+     unregistration performed by the kernel on thread teardown.  This is not a
+     problem because the rseq TLS lives on the stack, and the stack outlives
+     the thread.  If TCB allocation is ever changed, additional steps may be
+     required, such as performing explicit rseq unregistration before
+     reclaiming the rseq TLS area memory.  It is NOT sufficient to block
+     signals because the kernel may write to the rseq area even without
+     signals.
+
      The exit code is zero since in case all threads exit by calling
      'pthread_exit' the exit status must be 0 (zero).  */
   while (1)
diff --git a/sysdeps/nptl/dl-tls_init_tp.c b/sysdeps/nptl/dl-tls_init_tp.c
index ca494dd3a5..fedb876fdb 100644
--- a/sysdeps/nptl/dl-tls_init_tp.c
+++ b/sysdeps/nptl/dl-tls_init_tp.c
@@ -21,6 +21,7 @@ 
 #include <list.h>
 #include <pthreadP.h>
 #include <tls.h>
+#include <rseq-internal.h>
 
 #ifndef __ASSUME_SET_ROBUST_LIST
 bool __nptl_set_robust_list_avail;
@@ -57,11 +58,12 @@  __tls_pre_init_tp (void)
 void
 __tls_init_tp (void)
 {
+  struct pthread *pd = THREAD_SELF;
+
   /* Set up thread stack list management.  */
-  list_add (&THREAD_SELF->list, &GL (dl_stack_user));
+  list_add (&pd->list, &GL (dl_stack_user));
 
    /* Early initialization of the TCB.   */
-   struct pthread *pd = THREAD_SELF;
    pd->tid = INTERNAL_SYSCALL_CALL (set_tid_address, &pd->tid);
    THREAD_SETMEM (pd, specific[0], &pd->specific_1stblock[0]);
    THREAD_SETMEM (pd, user_stack, true);
@@ -90,6 +92,8 @@  __tls_init_tp (void)
       }
   }
 
+  rseq_register_current_thread (pd);
+
   /* Set initial thread's stack block from 0 up to __libc_stack_end.
      It will be bigger than it actually is, but for unwind.c/pt-longjmp.c
      purposes this is good enough.  */
diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
index 29c6c78f98..eb0f5fc021 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/sysdeps/unix/sysv/linux/Makefile
@@ -131,7 +131,10 @@  ifeq ($(have-GLIBC_2.27)$(build-shared),yesyes)
 tests += tst-ofdlocks-compat
 endif
 
-tests-internal += tst-sigcontext-get_pc
+tests-internal += \
+  tst-rseq \
+  tst-sigcontext-get_pc \
+  # tests-internal
 
 tests-time64 += \
   tst-adjtimex-time64 \
@@ -357,4 +360,8 @@  endif
 
 ifeq ($(subdir),nptl)
 tests += tst-align-clone tst-getpid1
+
+# tst-rseq-nptl is an internal test because it requires a definition of
+# __NR_rseq from the internal system call list.
+tests-internal += tst-rseq-nptl
 endif
diff --git a/sysdeps/unix/sysv/linux/aarch64/bits/rseq.h b/sysdeps/unix/sysv/linux/aarch64/bits/rseq.h
new file mode 100644
index 0000000000..9ba92725c7
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/aarch64/bits/rseq.h
@@ -0,0 +1,43 @@ 
+/* Restartable Sequences Linux aarch64 architecture header.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_RSEQ_H
+# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
+#endif
+
+/* RSEQ_SIG is a signature required before each abort handler code.
+
+   It is a 32-bit value that maps to actual architecture code compiled
+   into applications and libraries.  It needs to be defined for each
+   architecture.  When choosing this value, it needs to be taken into
+   account that generating invalid instructions may have ill effects on
+   tools like objdump, and may also have impact on the CPU speculative
+   execution efficiency in some cases.
+
+   aarch64 -mbig-endian generates mixed endianness code vs data:
+   little-endian code and big-endian data.  Ensure the RSEQ_SIG signature
+   matches code endianness.  */
+
+#define RSEQ_SIG_CODE  0xd428bc00  /* BRK #0x45E0.  */
+
+#ifdef __AARCH64EB__
+# define RSEQ_SIG_DATA 0x00bc28d4  /* BRK #0x45E0.  */
+#else
+# define RSEQ_SIG_DATA RSEQ_SIG_CODE
+#endif
+
+#define RSEQ_SIG       RSEQ_SIG_DATA
diff --git a/sysdeps/unix/sysv/linux/arm/bits/rseq.h b/sysdeps/unix/sysv/linux/arm/bits/rseq.h
new file mode 100644
index 0000000000..0542b26f6a
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/arm/bits/rseq.h
@@ -0,0 +1,83 @@ 
+/* Restartable Sequences Linux arm architecture header.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_RSEQ_H
+# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
+#endif
+
+/*
+   RSEQ_SIG is a signature required before each abort handler code.
+
+   It is a 32-bit value that maps to actual architecture code compiled
+   into applications and libraries.  It needs to be defined for each
+   architecture.  When choosing this value, it needs to be taken into
+   account that generating invalid instructions may have ill effects on
+   tools like objdump, and may also have impact on the CPU speculative
+   execution efficiency in some cases.
+
+   - ARM little endian
+
+   RSEQ_SIG uses the udf A32 instruction with an uncommon immediate operand
+   value 0x5de3.  This traps if user-space reaches this instruction by mistake,
+   and the uncommon operand ensures the kernel does not move the instruction
+   pointer to attacker-controlled code on rseq abort.
+
+   The instruction pattern in the A32 instruction set is:
+
+   e7f5def3    udf    #24035    ; 0x5de3
+
+   This translates to the following instruction pattern in the T16 instruction
+   set:
+
+   little endian:
+   def3        udf    #243      ; 0xf3
+   e7f5        b.n    <7f5>
+
+   - ARMv6+ big endian (BE8):
+
+   ARMv6+ -mbig-endian generates mixed endianness code vs data: little-endian
+   code and big-endian data.  The data value of the signature needs to have its
+   byte order reversed to generate the trap instruction:
+
+   Data: 0xf3def5e7
+
+   Translates to this A32 instruction pattern:
+
+   e7f5def3    udf    #24035    ; 0x5de3
+
+   Translates to this T16 instruction pattern:
+
+   def3        udf    #243      ; 0xf3
+   e7f5        b.n    <7f5>
+
+   - Prior to ARMv6 big endian (BE32):
+
+   Prior to ARMv6, -mbig-endian generates big-endian code and data
+   (which match), so the endianness of the data representation of the
+   signature should not be reversed.  However, the choice between BE32
+   and BE8 is done by the linker, so we cannot know whether code and
+   data endianness will be mixed before the linker is invoked.  So rather
+   than try to play tricks with the linker, the rseq signature is simply
+   data (not a trap instruction) prior to ARMv6 on big endian.  This is
+   why the signature is expressed as data (.word) rather than as
+   instruction (.inst) in assembler.  */
+
+#ifdef __ARMEB__
+# define RSEQ_SIG    0xf3def5e7      /* udf    #24035    ; 0x5de3 (ARMv6+) */
+#else
+# define RSEQ_SIG    0xe7f5def3      /* udf    #24035    ; 0x5de3 */
+#endif
diff --git a/sysdeps/unix/sysv/linux/bits/rseq.h b/sysdeps/unix/sysv/linux/bits/rseq.h
new file mode 100644
index 0000000000..46cf5d1c74
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/bits/rseq.h
@@ -0,0 +1,29 @@ 
+/* Restartable Sequences architecture header.  Stub version.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_RSEQ_H
+# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
+#endif
+
+/* RSEQ_SIG is a signature required before each abort handler code.
+
+   It is a 32-bit value that maps to actual architecture code compiled
+   into applications and libraries.  It needs to be defined for each
+   architecture.  When choosing this value, it needs to be taken into
+   account that generating invalid instructions may have ill effects on
+   tools like objdump, and may also have impact on the CPU speculative
+   execution efficiency in some cases.  */
diff --git a/sysdeps/unix/sysv/linux/mips/bits/rseq.h b/sysdeps/unix/sysv/linux/mips/bits/rseq.h
new file mode 100644
index 0000000000..a9defee568
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/mips/bits/rseq.h
@@ -0,0 +1,62 @@ 
+/* Restartable Sequences Linux mips architecture header.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_RSEQ_H
+# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
+#endif
+
+/* RSEQ_SIG is a signature required before each abort handler code.
+
+   It is a 32-bit value that maps to actual architecture code compiled
+   into applications and libraries.  It needs to be defined for each
+   architecture.  When choosing this value, it needs to be taken into
+   account that generating invalid instructions may have ill effects on
+   tools like objdump, and may also have impact on the CPU speculative
+   execution efficiency in some cases.
+
+   RSEQ_SIG uses the break instruction.  The instruction pattern is:
+
+   On MIPS:
+        0350000d        break     0x350
+
+   On nanoMIPS:
+        00100350        break     0x350
+
+   On microMIPS:
+        0000d407        break     0x350
+
+   For nanoMIPS32 and microMIPS, the instruction stream is encoded as
+   16-bit halfwords, so the signature halfwords need to be swapped
+   accordingly for little-endian.  */
+
+#if defined (__nanomips__)
+# ifdef __MIPSEL__
+#  define RSEQ_SIG      0x03500010
+# else
+#  define RSEQ_SIG      0x00100350
+# endif
+#elif defined (__mips_micromips)
+# ifdef __MIPSEL__
+#  define RSEQ_SIG      0xd4070000
+# else
+#  define RSEQ_SIG      0x0000d407
+# endif
+#elif defined (__mips__)
+# define RSEQ_SIG       0x0350000d
+#else
+/* Unknown MIPS architecture.  */
+#endif
diff --git a/sysdeps/unix/sysv/linux/powerpc/bits/rseq.h b/sysdeps/unix/sysv/linux/powerpc/bits/rseq.h
new file mode 100644
index 0000000000..05b3cf7b8f
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/powerpc/bits/rseq.h
@@ -0,0 +1,37 @@ 
+/* Restartable Sequences Linux powerpc architecture header.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_RSEQ_H
+# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
+#endif
+
+/* RSEQ_SIG is a signature required before each abort handler code.
+
+   It is a 32-bit value that maps to actual architecture code compiled
+   into applications and libraries.  It needs to be defined for each
+   architecture.  When choosing this value, it needs to be taken into
+   account that generating invalid instructions may have ill effects on
+   tools like objdump, and may also have impact on the CPU speculative
+   execution efficiency in some cases.
+
+   RSEQ_SIG uses the following trap instruction:
+
+   powerpc-be:    0f e5 00 0b           twui   r5,11
+   powerpc64-le:  0b 00 e5 0f           twui   r5,11
+   powerpc64-be:  0f e5 00 0b           twui   r5,11  */
+
+#define RSEQ_SIG        0x0fe5000b
diff --git a/sysdeps/unix/sysv/linux/rseq-internal.h b/sysdeps/unix/sysv/linux/rseq-internal.h
new file mode 100644
index 0000000000..909f547825
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/rseq-internal.h
@@ -0,0 +1,45 @@ 
+/* Restartable Sequences internal API.  Linux implementation.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef RSEQ_INTERNAL_H
+#define RSEQ_INTERNAL_H
+
+#include <sysdep.h>
+#include <errno.h>
+#include <kernel-features.h>
+#include <stdio.h>
+#include <sys/rseq.h>
+
+#ifdef RSEQ_SIG
+static inline void
+rseq_register_current_thread (struct pthread *self)
+{
+  int ret = INTERNAL_SYSCALL_CALL (rseq,
+                                   &self->rseq_area, sizeof (self->rseq_area),
+                                   0, RSEQ_SIG);
+  if (INTERNAL_SYSCALL_ERROR_P (ret))
+    THREAD_SETMEM (self, rseq_area.cpu_id, RSEQ_CPU_ID_REGISTRATION_FAILED);
+}
+#else /* RSEQ_SIG */
+static inline void
+rseq_register_current_thread (struct pthread *self)
+{
+  THREAD_SETMEM (self, rseq_area.cpu_id, RSEQ_CPU_ID_REGISTRATION_FAILED);
+}
+#endif /* RSEQ_SIG */
+
+#endif /* rseq-internal.h */
diff --git a/sysdeps/unix/sysv/linux/s390/bits/rseq.h b/sysdeps/unix/sysv/linux/s390/bits/rseq.h
new file mode 100644
index 0000000000..3030e38f40
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/s390/bits/rseq.h
@@ -0,0 +1,37 @@ 
+/* Restartable Sequences Linux s390 architecture header.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_RSEQ_H
+# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
+#endif
+
+/* RSEQ_SIG is a signature required before each abort handler code.
+
+   It is a 32-bit value that maps to actual architecture code compiled
+   into applications and libraries.  It needs to be defined for each
+   architecture.  When choosing this value, it needs to be taken into
+   account that generating invalid instructions may have ill effects on
+   tools like objdump, and may also have impact on the CPU speculative
+   execution efficiency in some cases.
+
+   RSEQ_SIG uses the trap4 instruction.  As Linux does not make use of the
+   access-register mode nor the linkage stack this instruction will always
+   cause a special-operation exception (the trap-enabled bit in the DUCT
+   is and will stay 0).  The instruction pattern is
+       b2 ff 0f ff        trap4   4095(%r0)  */
+
+#define RSEQ_SIG        0xB2FF0FFF
diff --git a/sysdeps/unix/sysv/linux/sys/rseq.h b/sysdeps/unix/sysv/linux/sys/rseq.h
new file mode 100644
index 0000000000..c8edff50d4
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/sys/rseq.h
@@ -0,0 +1,174 @@ 
+/* Restartable Sequences exported symbols.  Linux header.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_RSEQ_H
+#define _SYS_RSEQ_H	1
+
+/* Architecture-specific rseq signature.  */
+#include <bits/rseq.h>
+
+#include <stdint.h>
+#include <sys/cdefs.h>
+#include <bits/endian.h>
+
+#ifdef __has_include
+# if __has_include ("linux/rseq.h")
+#  define __GLIBC_HAVE_KERNEL_RSEQ
+# endif
+#else
+# include <linux/version.h>
+# if LINUX_VERSION_CODE >= KERNEL_VERSION (4, 18, 0)
+#  define __GLIBC_HAVE_KERNEL_RSEQ
+# endif
+#endif
+
+#ifdef __GLIBC_HAVE_KERNEL_RSEQ
+/* We use the structures declarations from the kernel headers.  */
+# include <linux/rseq.h>
+#else /* __GLIBC_HAVE_KERNEL_RSEQ */
+/* We use a copy of the include/uapi/linux/rseq.h kernel header.  */
+
+enum rseq_cpu_id_state
+  {
+    RSEQ_CPU_ID_UNINITIALIZED = -1,
+    RSEQ_CPU_ID_REGISTRATION_FAILED = -2,
+  };
+
+enum rseq_flags
+  {
+    RSEQ_FLAG_UNREGISTER = (1 << 0),
+  };
+
+enum rseq_cs_flags_bit
+  {
+    RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT_BIT = 0,
+    RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT = 1,
+    RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT = 2,
+  };
+
+enum rseq_cs_flags
+  {
+    RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT =
+      (1U << RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT_BIT),
+    RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL =
+      (1U << RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT),
+    RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE =
+      (1U << RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT),
+  };
+
+/* struct rseq_cs is aligned on 32 bytes to ensure it is always
+   contained within a single cache-line.  It is usually declared as
+   link-time constant data.  */
+struct rseq_cs
+  {
+    /* Version of this structure.  */
+    uint32_t version;
+    /* enum rseq_cs_flags.  */
+    uint32_t flags;
+    uint64_t start_ip;
+    /* Offset from start_ip.  */
+    uint64_t post_commit_offset;
+    uint64_t abort_ip;
+  } __attribute__ ((__aligned__ (32)));
+
+/* struct rseq is aligned on 32 bytes to ensure it is always
+   contained within a single cache-line.
+
+   A single struct rseq per thread is allowed.  */
+struct rseq
+  {
+    /* Restartable sequences cpu_id_start field.  Updated by the
+       kernel.  Read by user-space with single-copy atomicity
+       semantics.  This field should only be read by the thread which
+       registered this data structure.  Aligned on 32-bit.  Always
+       contains a value in the range of possible CPUs, although the
+       value may not be the actual current CPU (e.g. if rseq is not
+       initialized).  This CPU number value should always be compared
+       against the value of the cpu_id field before performing a rseq
+       commit or returning a value read from a data structure indexed
+       using the cpu_id_start value.  */
+    uint32_t cpu_id_start;
+    /* Restartable sequences cpu_id field.  Updated by the kernel.
+       Read by user-space with single-copy atomicity semantics.  This
+       field should only be read by the thread which registered this
+       data structure.  Aligned on 32-bit.  Values
+       RSEQ_CPU_ID_UNINITIALIZED and RSEQ_CPU_ID_REGISTRATION_FAILED
+       have a special semantic: the former means "rseq uninitialized",
+       and latter means "rseq initialization failed".  This value is
+       meant to be read within rseq critical sections and compared
+       with the cpu_id_start value previously read, before performing
+       the commit instruction, or read and compared with the
+       cpu_id_start value before returning a value loaded from a data
+       structure indexed using the cpu_id_start value.  */
+    uint32_t cpu_id;
+    /* Restartable sequences rseq_cs field.
+
+       Contains NULL when no critical section is active for the current
+       thread, or holds a pointer to the currently active struct rseq_cs.
+
+       Updated by user-space, which sets the address of the currently
+       active rseq_cs at the beginning of assembly instruction sequence
+       block, and set to NULL by the kernel when it restarts an assembly
+       instruction sequence block, as well as when the kernel detects that
+       it is preempting or delivering a signal outside of the range
+       targeted by the rseq_cs.  Also needs to be set to NULL by user-space
+       before reclaiming memory that contains the targeted struct rseq_cs.
+
+       Read and set by the kernel.  Set by user-space with single-copy
+       atomicity semantics.  This field should only be updated by the
+       thread which registered this data structure.  Aligned on 64-bit.  */
+    union
+      {
+        uint64_t ptr64;
+# ifdef __LP64__
+        uint64_t ptr;
+# else /* __LP64__ */
+        struct
+          {
+#if __BYTE_ORDER == __BIG_ENDIAN
+            uint32_t padding; /* Initialized to zero.  */
+            uint32_t ptr32;
+#  else /* LITTLE */
+            uint32_t ptr32;
+            uint32_t padding; /* Initialized to zero.  */
+#  endif /* ENDIAN */
+          } ptr;
+# endif /* __LP64__ */
+      } rseq_cs;
+
+    /* Restartable sequences flags field.
+
+       This field should only be updated by the thread which
+       registered this data structure.  Read by the kernel.
+       Mainly used for single-stepping through rseq critical sections
+       with debuggers.
+
+       - RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
+           Inhibit instruction sequence block restart on preemption
+           for this thread.
+       - RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
+           Inhibit instruction sequence block restart on signal
+           delivery for this thread.
+       - RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
+           Inhibit instruction sequence block restart on migration for
+           this thread.  */
+    uint32_t flags;
+  } __attribute__ ((__aligned__ (32)));
+
+#endif /* __GLIBC_HAVE_KERNEL_RSEQ */
+
+#endif /* sys/rseq.h */
diff --git a/sysdeps/unix/sysv/linux/tst-rseq-nptl.c b/sysdeps/unix/sysv/linux/tst-rseq-nptl.c
new file mode 100644
index 0000000000..d31d94445c
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-rseq-nptl.c
@@ -0,0 +1,260 @@ 
+/* Restartable Sequences NPTL test.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* These tests validate that rseq is registered from various execution
+   contexts (main thread, destructor, other threads, other threads created
+   from destructor, forked process (without exec), pthread_atfork handlers,
+   pthread setspecific destructors, signal handlers, atexit handlers).
+
+   See the Linux kernel selftests for extensive rseq stress-tests.  */
+
+#include <stdio.h>
+#include <support/check.h>
+#include <support/xthread.h>
+#include <sys/rseq.h>
+#include <unistd.h>
+
+#ifdef RSEQ_SIG
+# include <array_length.h>
+# include <errno.h>
+# include <error.h>
+# include <pthread.h>
+# include <signal.h>
+# include <stdlib.h>
+# include <string.h>
+# include <support/namespace.h>
+# include <support/xsignal.h>
+# include <syscall.h>
+# include <sys/types.h>
+# include <sys/wait.h>
+# include "tst-rseq.h"
+
+static pthread_key_t rseq_test_key;
+
+static void
+atfork_prepare (void)
+{
+  if (!rseq_thread_registered ())
+    {
+      printf ("error: rseq not registered in pthread atfork prepare\n");
+      support_record_failure ();
+    }
+}
+
+static void
+atfork_parent (void)
+{
+  if (!rseq_thread_registered ())
+    {
+      printf ("error: rseq not registered in pthread atfork parent\n");
+      support_record_failure ();
+    }
+}
+
+static void
+atfork_child (void)
+{
+  if (!rseq_thread_registered ())
+    {
+      printf ("error: rseq not registered in pthread atfork child\n");
+      support_record_failure ();
+    }
+}
+
+static void
+rseq_key_destructor (void *arg)
+{
+  /* Cannot use deferred failure reporting after main returns.  */
+  if (!rseq_thread_registered ())
+    FAIL_EXIT1 ("rseq not registered in pthread key destructor");
+}
+
+static void
+atexit_handler (void)
+{
+  /* Cannot use deferred failure reporting after main returns.  */
+  if (!rseq_thread_registered ())
+    FAIL_EXIT1 ("rseq not registered in atexit handler");
+}
+
+/* Used to avoid -Werror=stringop-overread warning with
+   pthread_setspecific and GCC 11.  */
+static char one = 1;
+
+static void
+do_rseq_main_test (void)
+{
+  TEST_COMPARE (atexit (atexit_handler), 0);
+  rseq_test_key = xpthread_key_create (rseq_key_destructor);
+  TEST_COMPARE (pthread_atfork (atfork_prepare, atfork_parent, atfork_child), 0);
+  xraise (SIGUSR1);
+  TEST_COMPARE (pthread_setspecific (rseq_test_key, &one), 0);
+  TEST_VERIFY_EXIT (rseq_thread_registered ());
+}
+
+static void
+cancel_routine (void *arg)
+{
+  if (!rseq_thread_registered ())
+    {
+      printf ("error: rseq not registered in cancel routine\n");
+      support_record_failure ();
+    }
+}
+
+static pthread_barrier_t cancel_thread_barrier;
+static pthread_cond_t cancel_thread_cond = PTHREAD_COND_INITIALIZER;
+static pthread_mutex_t cancel_thread_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+static void
+test_cancel_thread (void)
+{
+  pthread_cleanup_push (cancel_routine, NULL);
+  (void) xpthread_barrier_wait (&cancel_thread_barrier);
+  /* Wait forever until cancellation.  */
+  xpthread_cond_wait (&cancel_thread_cond, &cancel_thread_mutex);
+  pthread_cleanup_pop (0);
+}
+
+static void *
+thread_function (void * arg)
+{
+  int i = (int) (intptr_t) arg;
+
+  xraise (SIGUSR1);
+  if (i == 0)
+    test_cancel_thread ();
+  TEST_COMPARE (pthread_setspecific (rseq_test_key, &one), 0);
+  return rseq_thread_registered () ? NULL : (void *) 1l;
+}
+
+static void
+sighandler (int sig)
+{
+  if (!rseq_thread_registered ())
+    {
+      printf ("error: rseq not registered in signal handler\n");
+      support_record_failure ();
+    }
+}
+
+static void
+setup_signals (void)
+{
+  struct sigaction sa;
+
+  sigemptyset (&sa.sa_mask);
+  sigaddset (&sa.sa_mask, SIGUSR1);
+  sa.sa_flags = 0;
+  sa.sa_handler = sighandler;
+  xsigaction (SIGUSR1, &sa, NULL);
+}
+
+static int
+do_rseq_threads_test (int nr_threads)
+{
+  pthread_t th[nr_threads];
+  int i;
+  int result = 0;
+
+  xpthread_barrier_init (&cancel_thread_barrier, NULL, 2);
+
+  for (i = 0; i < nr_threads; ++i)
+    th[i] = xpthread_create (NULL, thread_function,
+                             (void *) (intptr_t) i);
+
+  (void) xpthread_barrier_wait (&cancel_thread_barrier);
+
+  xpthread_cancel (th[0]);
+
+  for (i = 0; i < nr_threads; ++i)
+    {
+      void *v;
+
+      v = xpthread_join (th[i]);
+      if (i != 0 && v != NULL)
+        {
+          printf ("error: join %d successful, but child failed\n", i);
+          result = 1;
+        }
+      else if (i == 0 && v == NULL)
+        {
+          printf ("error: join %d successful, child did not fail as expected\n", i);
+          result = 1;
+        }
+    }
+
+  xpthread_barrier_destroy (&cancel_thread_barrier);
+
+  return result;
+}
+
+static void
+subprocess_callback (void *closure)
+{
+  do_rseq_main_test ();
+}
+
+static void
+do_rseq_fork_test (void)
+{
+  support_isolate_in_subprocess (subprocess_callback, NULL);
+}
+
+static int
+do_rseq_test (void)
+{
+  int t[] = { 1, 2, 6, 5, 4, 3, 50 };
+  int i, result = 0;
+
+  if (!rseq_available ())
+    FAIL_UNSUPPORTED ("kernel does not support rseq, skipping test");
+  setup_signals ();
+  xraise (SIGUSR1);
+  do_rseq_main_test ();
+  for (i = 0; i < array_length (t); i++)
+    if (do_rseq_threads_test (t[i]))
+      result = 1;
+  do_rseq_fork_test ();
+  return result;
+}
+
+static void __attribute__ ((destructor))
+do_rseq_destructor_test (void)
+{
+  /* Cannot use deferred failure reporting after main returns.  */
+  if (do_rseq_test ())
+    FAIL_EXIT1 ("rseq not registered within destructor");
+  xpthread_key_delete (rseq_test_key);
+}
+
+#else /* RSEQ_SIG */
+static int
+do_rseq_test (void)
+{
+  FAIL_UNSUPPORTED ("glibc does not define RSEQ_SIG, skipping test");
+  return 0;
+}
+#endif /* RSEQ_SIG */
+
+static int
+do_test (void)
+{
+  return do_rseq_test ();
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/unix/sysv/linux/tst-rseq.c b/sysdeps/unix/sysv/linux/tst-rseq.c
new file mode 100644
index 0000000000..926376b6a5
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-rseq.c
@@ -0,0 +1,64 @@ 
+/* Restartable Sequences single-threaded tests.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* These tests validate that rseq is registered from main in an executable
+   not linked against libpthread.  */
+
+#include <support/check.h>
+#include <stdio.h>
+#include <sys/rseq.h>
+#include <unistd.h>
+
+#ifdef RSEQ_SIG
+# include <errno.h>
+# include <error.h>
+# include <stdlib.h>
+# include <string.h>
+# include <syscall.h>
+# include "tst-rseq.h"
+
+static void
+do_rseq_main_test (void)
+{
+  TEST_VERIFY_EXIT (rseq_thread_registered ());
+}
+
+static void
+do_rseq_test (void)
+{
+  if (!rseq_available ())
+    {
+      FAIL_UNSUPPORTED ("kernel does not support rseq, skipping test");
+    }
+  do_rseq_main_test ();
+}
+#else /* RSEQ_SIG */
+static void
+do_rseq_test (void)
+{
+  FAIL_UNSUPPORTED ("glibc does not define RSEQ_SIG, skipping test");
+}
+#endif /* RSEQ_SIG */
+
+static int
+do_test (void)
+{
+  do_rseq_test ();
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/sysdeps/unix/sysv/linux/tst-rseq.h b/sysdeps/unix/sysv/linux/tst-rseq.h
new file mode 100644
index 0000000000..12d4639874
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-rseq.h
@@ -0,0 +1,57 @@ 
+/* Restartable Sequences tests header.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <error.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <support/check.h>
+#include <syscall.h>
+#include <sys/rseq.h>
+#include <tls.h>
+
+static inline bool
+rseq_thread_registered (void)
+{
+  return THREAD_GETMEM (THREAD_SELF, rseq_area.cpu_id) >= 0;
+}
+
+static inline int
+sys_rseq (struct rseq *rseq_abi, uint32_t rseq_len, int flags, uint32_t sig)
+{
+  return syscall (__NR_rseq, rseq_abi, rseq_len, flags, sig);
+}
+
+static inline bool
+rseq_available (void)
+{
+  int rc;
+
+  rc = sys_rseq (NULL, 0, 0, 0);
+  if (rc != -1)
+    FAIL_EXIT1 ("Unexpected rseq return value %d", rc);
+  switch (errno)
+    {
+    case ENOSYS:
+      return false;
+    case EINVAL:
+      /* rseq is implemented, but detected an invalid rseq_len parameter.  */
+      return true;
+    default:
+      FAIL_EXIT1 ("Unexpected rseq error %s", strerror (errno));
+    }
+}
diff --git a/sysdeps/unix/sysv/linux/x86/bits/rseq.h b/sysdeps/unix/sysv/linux/x86/bits/rseq.h
new file mode 100644
index 0000000000..9fc909e7c8
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86/bits/rseq.h
@@ -0,0 +1,30 @@ 
+/* Restartable Sequences Linux x86 architecture header.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_RSEQ_H
+# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
+#endif
+
+/* RSEQ_SIG is a signature required before each abort handler code.
+
+   RSEQ_SIG is used with the following reserved undefined instructions, which
+   trap in user-space:
+
+   x86-32:    0f b9 3d 53 30 05 53      ud1    0x53053053,%edi
+   x86-64:    0f b9 3d 53 30 05 53      ud1    0x53053053(%rip),%edi  */
+
+#define RSEQ_SIG        0x53053053